But those are the big-name review sites—just the tip of the tip of the iceberg. For everything you might spend money on, somebody has set up a Web site where fans and foes can exchange reviews. Here are just a few:

• Recipes (Epicurious.com). Here it is, one of the Internet’s most useful clearinghouses of recipes. Recipes are backed up by photos, videos, a place for you own notes—and reviews by people who’ve made the recipes in question. (“Well, I have to agree with all other reviews. This was without doubt the best roast chicken I ever cooked. I am now wondering if it would work for turkey.”)

• Musical instruments (Harmonycentral.com). Click "User Reviews" to read the tales of veterans who’ve already bought these horns, drums, guitars, keyboards and headphones, so you’ll know what you’re getting into. (“The action is heavy, much heavier than many of the newer Digital Pianos; it [weighs]a ton…. It needs a Z-stand or it would just break the X-stand.”)

• Online stores (ResellerRatings.com). Suppose some Web store is offering you a great price on something. How do you know if you’re in for some grey-market scariness? Check here first to make sure the store has a good track record. (“I canceled an order before it even shipped…and was not given a refund. After long hours on hold, dealing with rude and dismissive customer service, and just plain being ignored, I finally left it to my credit-card company to deal with.”)

• Movies on DVD (Netflix.com). Most people think of Netflix as a place to rent DVDs by mail (or stream movies), but the consumer ratings of the movies are often more literate and thoughtful than those at the big-name movie-ratings sites like IMDB and Rotten Tomatoes. (“A house lifted from its foundation by an enormous collection of balloons. If you can suspend your disbelief long enough to get past that concept, then you have a real treat on your hands.”)

• Makeup (MakeupAlley.com). Find find out how good some makeup product is before you shell out for it. (“I am in love with this foundation. [It] blends onto my skin so easily and sinks in like no other, it gives the most healthy and beautiful glow without [looking] oily…. [Ask] for a sample to see if you like it first, because this foundation is verrrrrrrrrrrrrrry expensive.”)

• Doctors (ZocDoc.com). There are plenty of doctor-rating sites, but many of them stir up only people who’ve had bad experiences. ZocDoc accepts reviews only from patients who’ve actually seen the doctors. It also lets you book an appointment with one click. Available in only five major cities so far. (“He is gentle, remembers [my children’s] names, does not rush, speaks to us appropriately, and returns his own calls. He is truly a class act and a spectacular doctor all around.”)

• Frequent-flyer programs (Flyertalk.com). Yes, there’s even a site where people rate frequent-flyer programs. ‘Nuff said. (“Delta offers award seats at more than one redemption level. A business class transatlantic trip requires 50,000, 100,000 or 162,500 miles in each direction.”)